The nil dereference in the next few lines doesn't seem
to cause a segmentation fault on Unix, but does seem
to halt the Go compiler.

The following is a test case:

>>>
package main

func mine(int b) int {
        return b + 2
}

func main() {
        mine()

        c = mine()
}
<<<

Without this change only the following is caught:

typecheck.go:3: undefined: b
typecheck.go:4: undefined: b

with it, we catch all the errors:

typecheck.go:3: undefined: b
typecheck.go:4: undefined: b
typecheck.go:10: undefined: c
typecheck.go:10: cannot assign to c .

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6542060